Method and apparatus for surface treatment using inorganic acid and ozone

ABSTRACT

Improved removal of ion-implanted photoresist in a single wafer front-end wet processing station is achieved by dissolving gaseous ozone into relatively cool inorganic acid, dispensing the acid ozone mixture onto a wafer, and rapidly heating the surface of the wafer to a temperature at least 30° C. higher than the temperature of the acid ozone mixture.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to methods and apparatus for treating surfaces ofarticles, such as semiconductor wafers, using combinations of inorganicacid and ozone.

2. Description of Related Art

Semiconductor wafers undergo a variety of wet processing stages duringmanufacture of integrated circuits, one of which is removal ofphotoresist from the wafer. When the photoresist is stripped by a wetprocess, among the techniques used for that stripping is a SOM (sulfuricacid ozone mixture) process. These processes involve dissolving ozone insulfuric acid so that the ozone reacts with the sulfuric acid to formdipersulfuric acid or peroxydisulfuric acid (H₂S₂O₈), as shown by thefollowing equation:2HSO₄ ⁻+O₃<===>O₂+H₂O+S₂O₈ ²⁻

Ozone that does not react with sulfuric acid can also dissolve as suchinto the sulfuric acid solution, and thus serve as an oxidizing agentfor the material to be stripped.

SOM processes typically dissolve ozone into hot sulfuric acid (110 to150° C.) before the mixture is dispensed onto a semiconductor wafer fora certain amount of time.

The present inventors have identified a number of disadvantages of theconventional SOM processes, including that the ozone has a significantlylower solubility in sulfuric acid at such high temperatures and secondalso a very short half life (the half-life of ozone in sulfuric acid at150° C. has been found to be less than 10 seconds). Consequently, onlysmall amounts of ozone and/or reactive peroxydisulfate ion reach thewafer and thus photoresist strip rates are relatively low, leading tolong process times, low throughput and with this high cost-of-ownership.

In addition a continuous dispensing of the mixture is required for thewhole process time in order to ensure a continuous supply of ozone andits reaction products to the wafer surface. For long process times thisalso results in a high chemical cost in terms of sulfuric acid, despitethe ability to generate ozone in situ.

Still further, if only one chemical supply system is being used, whichserves more than one process chamber, the distances for the chemicaldistribution lines are typically different to the individual processchambers, leading to a chamber-to-chamber variability in the actualozone concentration delivered to each wafer.

Previous attempts to improve SOM processes have included dispensingsulfuric acid at a temperature in the range of about 25-150° C. onto thewafer and then providing ozone into the environment around the wafer(U.S. Pat. No. 6,869,487). In this way the ozone has to diffuse throughthe sulfuric acid layer to reach the photoresist to be decomposed. Thediffusion process is however a rather slow process even when only a thinliquid layer is being created, and so most of the active decompositionproducts from ozone (active radicals with a very short lifetime) cannotreach the photo resist in sufficiently large quantities, leading to anon-optimum strip rate.

It has also been proposed to pressurize the heated liquid and theninject gaseous ozone to increase the concentration of the active speciesin the liquid (EP 1 100 630 B1, U.S. Pat. No. 5,971,368, U.S. Pat. No.6,488,271). However, as soon as the pressure on the oversaturatedozonated liquid is reduced, during dispensing of the liquid onto thewafer, bubbles will form as the ozone comes out of solution, causingsignificant loss in the ozone concentration. Another limitation of thistechnique is that a pressurized chemical mixing system is required,which adds significantly to cost and raises safety concerns.

SUMMARY OF THE INVENTION

The method and apparatus of the present invention involves combiningozone and inorganic acid at relatively low temperatures, the ozoneinorganic acid mixture being present on a surface of an article to betreated, to form a liquid layer on the surface of the article to betreated, followed by heating the surface of the article to be treatedduring or after dispensing the inorganic acid, to a temperature at least30° C. greater than the temperature of the inorganic acid. In preferredembodiments, the surface of the article to be treated is heated to atemperature of at least 50° C., and more preferably at least 70° C., inexcess of the temperature of the inorganic acid.

In preferred embodiments the inorganic acid and ozone are combined toform an acid-ozone mixture before the acid is dispensed onto the articleto be treated. In other embodiments, the acid is first dispensed ontothe surface of the article to be treated, and ozone is present ingaseous form within the process chamber, where it passes by diffusionthrough the layer of inorganic acid formed on the surface of thearticle.

In some embodiments of the invention the inorganic acid is sulfuricacid, and the temperature of the sulfuric acid ozone mixture may beachieved by actively cooling sulfuric acid to a temperature below about20° C. in order to increase the dissolved ozone concentration in theliquid, which then is dispensed onto the photoresist-containing wafer.In such a way a very high ozone concentration is achieved directlywithin the boundary layer at the wafer surface. This method andapparatus permits improving the photoresist removal rate without theneed for difficult to handle and maintain equipment such as pressurizedmixing systems.

During or directly after dispensing of the inorganic acid onto the wafersurface the liquid and/or the wafer is heated to a temperature above 50°C., preferably above 100° C., and more preferably above 150° C., inorder to rapidly decompose the ozone within the boundary layer at thewafer surface.

Many means for heating the wafer can be applied, non-limiting examplesof which being IR radiation, steam, and a heated chuck/wafer support.

In another embodiment, the ozone decomposition and with this the radicalformation can also be initiated through other means like UV radiation.The result of the ozone decomposition, initiated within the liquidboundary layer at the wafer surface is that very active radicalintermediates are being created directly at the spot where they arerequired. In this way radicals with extremely short half-lifes and otheractive decomposition intermediates can react immediately with the photoresist and any residual photo resist crust layer.

Photo resist crust layers are very difficult to remove and are presentafter specific physical treatments of the resist layers, which are verycommon in the semiconductor industry. This is especially so when thephoto resist has previously undergone relatively high rates of ionimplantation, for example during doping of the wafer with for exampleboron or arsenic, which makes the subsequent stripping more difficult toachieve.

The methods and apparatus of the invention are not limited to use onsemiconductor wafers, and have application as well for treating surfacesof other materials, for example glass masters and mother panels used inmanufacturing optical disks and LCD display panels, as well as forcleaning surfaces of processing chambers used during processing of theabove-described substrates.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the invention will become moreapparent after reading the following detailed description of preferredembodiments of the invention, given with reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic representation of an apparatus for treatingsurfaces of semiconductor wafers according to an embodiment of theinvention;

FIG. 2 is a flow chart outlining several steps of a method for treatingsurfaces of semiconductor wafers according to an embodiment of theinvention;

FIG. 3 a shows an embodiment of a spin chuck suitable for use in themethod and apparatus according to the invention;

FIG. 3 b shows another embodiment of a spin chuck suitable for use inthe method and apparatus according to the invention; and

FIG. 4 shows yet another embodiment of a spin chuck suitable for use inthe method and apparatus according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The term “ozone” as used herein refers not only to ozone in the form ofgaseous O₃, but also ozone combined with other gases required forgenerating ozone, such as oxygen, nitrogen or carbon dioxide; ozone asdissolved in a strong inorganic acid such as sulfuric acid; as well asthe highly oxidative radicals formed by reaction of ozone with theinorganic acid.

In one embodiment of the method according to the present invention,sulfuric acid is chilled to below 20° C. for example in a tank, and iscontacted with ozone gas so as to allow ozone to dissolve in thesulfuric acid. Pure sulfuric acid has a melting point of 10.36° C.,whereas 98% sulfuric acid has a melting point of 3.0° C. the temperatureto which the sulfuric acid or other inorganic acid is chilled will notbe less than its melting point or that of the aqueous solution thereofbeing used.

However, the temperature at which the acid/ozone mixture is applied tothe wafer can be generated can also be a lot higher, for example 35° C.or even higher, provided that the differential between the temperatureof the mixture and the temperature to which the substrate is heated issufficiently large to provide rapid heating.

Furthermore, the ozone and inorganic acid may be combined either bymixing prior to dispensing of the acid onto the article (substrate), or,alternatively, the acid may be dispensed onto the article surface in aprocess chamber supplied with ozone gas, such that a thin layer of acidis formed on the substrate surface through which the gaseous ozone maydiffuse.

Whereas it is known that ozone is more soluble in water at lowertemperatures than at higher temperatures, the solubility of ozone ininorganic acids, and especially in sulfuric acid, has not beenconsidered to be a limiting factor in the field of resist stripping,since the ozone is also reactive with the acid to form, in the case ofsulfuric acid, the highly oxidative radicals described above. See, e.g.,U.S. Pat. No. 6,032,682 at col. 3, lines 6-31.

Thus the uptake of ozone by the acid exceeds the true dissolution ofozone as ozone into the acid to the extent of the occurrence suchchemical reactions. The use of cooled inorganic acid according tocertain embodiments of the present invention serves not only to increasethe concentration of unreacted ozone dissolved in the acid, but also totime-shift the chemical reactions between the ozone and the acid, as toat least a part of the introduced ozone, and permit the generation ofthe oxidative radicals at the location where their action is desired,the surface of the wafer to be treated.

For contacting the sulfuric acid with ozone gas, in the case of mixingthe two prior to dispensing, a variety of techniques and equipment canbe used either singly or in combination, e.g. a static mixer, stirringof the solution, and bubbling of ozone through the acid.

When the sulfuric acid is saturated with ozone at a given temperature,or when the sulfuric acid contains a desired concentration of ozonebelow saturation, the sulfuric acid/ozone mixture is dispensed insufficient quantities to cover the wafer surface onto a spinning wafer.Preferably, the sulfuric acid/ozone mixture is dispensed in an amountfrom about 10 to about 100 ml. The wafer is preferably positioned in aclosed process chamber of a single wafer wet processing station, andadditional ozone gas is preferably supplied to the chamber.

By keeping the spinning wafer enclosed in an environmentallycontrollable chamber system, the additional ozone gas with which thechamber is filled can be controlled to keep the ozone concentration inthe liquid layer on top of the wafer at a constant predetermined levelthrough diffusion.

The thickness of the liquid layer on the wafer can be adjusted throughthe amount of liquid being dispensed and by adjusting the wafer spinspeed.

After the ozone-containing sulfuric acid is dispensed onto the wafer andthe liquid layer thickness is adjusted, the liquid and/or the wafer isheated to a set temperature above 50° C., preferably above 100° C., andmore preferably above 150° C., in order to initiate the ozonedecomposition and radical formation, which then leads to a fastcracking, dissolution or decomposition of the photo resist and/or crustlayer.

Once the photo resist and/or crust has been partially or completelyremoved from the surface, depending on the required process goal, astream of ammonia gas can be introduced into the process chamber aroundthe sulfuric acid/ozone wetted wafer in order to actively react allsulfate on the surface to form ammonium sulfate and/or ammonium hydrogensulfate, which then can be easily rinsed off the wafer with DI water ordilute hydrochloric acid or any other chemistry capable of dissolvingthose salts sufficiently quickly and quantitatively. It may also bedesirable in some process to purge any excess ozone gas around the waferwith e.g. nitrogen or purified air before ammonia gas is being broughtin the environment around the wafer. Instead of ammonia gas also adilute ammonia-water solution can be applied for reacting the residualsulfates to the equivalent ammonium salts. When diluted hydrochloricacid is being applied for the removal of the sulfate salts, the processmight be followed with a DI water rinse that is approximately pH neutralor slightly acidified with dissolved carbon dioxide.

In FIG. 1 a 300 mm diameter semiconductor wafer is held by a spin chuck1, in a surrounding processing chamber C for single wafer wetprocessing. In this embodiment, a dispenser 2 of treatment liquidcomprises a dispense arm 3 with a dispense nozzle 4 configured todispense the treatment liquid onto the wafer in a free flow. The nozzleorifice has a cross-sectional area in the range of 3 to 300 mm², andpreferably 10 to 100 mm².

The treatment liquid is created by combining a cooled inorganic acid,preferably sulfuric acid, and gaseous ozone. In this embodiment, ozonegenerator 9 supplies gaseous ozone through pipe 6 into sulphuric acidtank 8, where the ozone bubbles through the acid.

The apparatus of this embodiment also includes a cooler 13 for coolingthe inorganic acid in tank 8. In practice the cooler 13 and tank 8 canbe embodied as a refrigerated tank. In this embodiment the inorganicacid is sulphuric acid, and the cooler 13 cools the acid to atemperature in a range of 3° C. to less than 20° C., preferably 5° C. to18° C., and more preferably 10° C. to 15° C. Alternatively or inaddition, an in-line cooler can be provided downstream of the outlet oftank 8, along the supply conduit from tank 8 or on the dispense arm 3.

In this embodiment an electrical resistance heater 15 heats the chuckand in turn the wafer either while the cooled SOM liquid is applied tothe wafer surface or after the liquid has been applied.

The reference herein to inorganic acids and sulphuric acid is intendedto encompass aqueous solutions of such acids, although it is preferredthat such solutions are nevertheless relatively concentrated, namely, aninitial concentration of at least 80 mass % and preferably of at least90 mass %. In the case of sulphuric acid, use can be made ofconcentrated sulphuric acid, having a mass percent of 98.3%.

The apparatus of this embodiment also includes a liquid collector 14 asis known in the art, wherein the liquid can be collected after beingspun off a rotating wafer, as well as a recycling system 16 wherein theremaining liquid is returned to a process tank.

A flow controller 17 includes a flow meter for measuring the flow in theliquid line, and can adjust the rate of flow to a desired value.

Dispense nozzle 4 in this embodiment preferably has a cross-sectionalarea of approximately ¼″ and may be formed of plural ⅛″ tubes joined toa single ½″ tube.

The wafer W is preferably rotating as the treatment liquid is dispensedonto it, and the rotational speed of the wafer is in the range of 0-1000rpm, preferably 30-300 rpm, preferably at a speed varying over time. Theinorganic acid/ozone mixture is supplied at a volumetric flow in therange of 0.5 to 5 liter per minute (lpm), preferably 0.7-2 lpm.

The concentration of the inorganic acid preferably ranges from about 80to about 98 mass %, which in the case of sulphuric acid includesconcentrated sulphuric acid at about 98.3% purity. More preferably, theconcentration of the inorganic acid is at least 90 mass %.

Oxidizing gas supply 9 is preferably an ozone generator. In this regard,and as is known to those skilled in the art, ozone (O₃) is ordinarilynot provided as a pure gas but rather is produced by reacting pureoxygen for example by silent electrostatic discharge, such that thegenerated ozone comprises oxygen (O₂) at a mass % of about 80 to about98% and ozone in a range of about 1-20 mass %. Reference herein to ozonegas includes such ozone-enriched oxygen gases.

Dispense arm 3 may be configured to operate as a boom swing, and thusmove horizontally relative to and across the rotating wafer. The speedand range of the boom swing movement is sufficiently wide and fast as topromote an even temperature distribution of the treatment liquid acrossthe wafer surface, thereby to improve uniformity of treatment over awafer surface.

In FIG. 2, a wafer first undergoes optional pretreatment in Step S1 suchas wetting to promote the contact and flow properties of the treatmentliquid on the wafer surface. Next, the inorganic acid/ozone mixture isdispensed onto the wafer surface in Step S2. The wafer W may be rotatedat the rpm described above during either or both of Steps S1 and S2.

When using H₂SO₄, the dwell time of the treatment liquid on a 300 mmdiameter semiconductor wafer is preferably about 30-240 sec, with atotal treatment time (i.e., including any prewet and rinse steps) ofabout 90-420 sec.

In Step S3, the thickness of the layer of treatment liquid is measuredby a suitable optical measuring device to determine whether the measuredthickness h equals a target thickness h_(set). The flow rate oftreatment liquid and spin speed of the chuck are adjusted until thetarget thickness is achieved.

In Step S4 the wafer is heated to the desired temperature toinitiate/accelerate reaction of the ozone at the wafer interface, andthe process is continued until a desired level of stripping has beenachieved. The supply of treatment liquid is terminated in Step S5.

As described above, during and following treatment the liquid iscollected and returned to the process tank 8, from which it isresupplied to the chamber C.

As the treatment liquid is recovered and recycled, the acid strengthwill gradually decline after a number of treatment cycles. The acidstrength may be restored by addition of fresh acid to the tank 8;alternatively or in addition, the oxidizing power of the treatmentliquid can be increased by adding H₂O₂ to the tank 8.

By partially draining the collector 14 over successive processingcycles, it can be possible to avoid the need to empty supply tank 8. Inparticular, the acid supply tank 8 can be kept in continuous servicewhen part of the recovered liquid is drained from collector 14 and partis recirculated to tank 8. After treatment of the wafer with theoxidizing liquid is complete, an optional rinse of the wafer isperformed in Step S6.

FIG. 3 a shows a spin chuck based on the designs described in WO2010/113089A2. The device 300 comprises a chamber, an annular chuck 320for gripping and rotating a wafer (disc-like article) and a stator 390.The chamber comprises a cylindrical wall, a bottom plate 365 and a topplate (not shown). An upper dispensing tube 363 is led through the topplate and supplies sulfuric acid to the wafer surface to be treated anda peripheral inlet 364 supplies ozone into the chamber. Lower dispensingtube 367 may provide additional process liquid or gas, or a water rinse.

The stator 390 is mounted to a stator base plate 305 concentric to thecylindrical wall 360. The stator base plate 305 can be moved in axialdirection with respect to the cylinder axis of the cylindrical wall 360,e.g. with pneumatic lifting means. The stator base plate 305 and thestator 390 mounted thereto have central openings, which have biggerdiameter than the outer diameter of the cylindrical wall 360. The topplate can also be moved in axial direction in order to open the chamber.In closed position the top plate is sealed against the cylindrical wall.

The stator 390 comprises several coils for axial and radial bearing andfor driving the rotor 385, which is part of the annular chuck. Sucharrangement is called active bearing and is further described in U.S.Pat. No. 6,485,531.

In FIG. 3 b, the spin chuck has been adapted to include fiveconcentrically arranged annular infra red (IR) heaters 381, 382, 383,384, 385. The IR-heaters are separately tunable, so as to achieve adesired uniformity of heating across the wafer surface. In other wordsif it is found out that the edge is not heated fast enough the power tothe outermost annular IR-heater 381 is increased. The tuning of theIR-heaters can be optimized by monitoring the treated wafers regardingthe uniformity of heating or by monitoring the temperature increaseusing local thermometers.

The IR-heaters are preferably annular quartz rods. There is an isolativecoating provided so that the IR-heaters primarily emit IR-light towardsthe wafer surface. The chamber and the IR-heaters are separated by atransparent plate 380, e.g. made of heat resistant glass (borosilicateglass), or quartz.

The assembly of IR heaters 381-385 and transparent plate 380 are mountedin the undepicted top plate of the chamber, and are thus in the positionshown when the chamber is closed.

In the alternative of FIG. 4, the transparent plate 480 is formed inorder to provide a much smaller distance (below 10 mm and preferablybelow 5 mm) between the transparent plate 480 and the wafer W. Inparticular, the transparent plate 480 of this embodiment has athree-dimensional shape in which the peripheral portion is at a higherlevel so as to provide clearance for the ring 320, whereas the centralportion is at a lower level, and joined to the higher portion by acylindrical wall, so that the central portion of plate 480 is positionedmuch closer to wafer W than in the embodiment of FIG. 3 b. The higherperipheral portion of plate 480 is joined to the undepicted top plate ofthe chamber, whereas the lower central portion of plate 480 dependsdownwardly therefrom.

Those skilled in the art will recognize that a variety ofthree-dimensional configurations for the transparent plate will beappropriate depending upon the particular structure of the chuck towhich the plate is applied. Alternatively, the ring 320 may be formedwith plate 480 secured thereto, in which case plate 480 need not have athree-dimensional shape, or in which case the height differentialbetween the peripheral and central regions of plate 480 could bereduced.

Although the heating elements in the foregoing embodiments areconcentric rings, the heating elements may alternatively be embodied astunable spot type IR lamps or tunable linear heating elements.

While the present invention has been described in connection withvarious preferred embodiments thereof, it is to be understood that thoseembodiments are provided merely to illustrate the invention, and shouldnot be used as a pretext to limit the scope of protection conferred bythe true scope and spirit of the appended claims.

What is claimed is:
 1. A method for treating a surface of an article,comprising: contacting ozone and an inorganic acid at a sub-ambienttemperature in a range of from about 3° C. to less than 20° C., to forma liquid mixture comprising ozone dissolved in the inorganic acid;dispensing said liquid mixture onto a surface of an article to betreated, said liquid mixture having a first temperature; forming aliquid layer of said liquid mixture on the surface of the article to betreated; heating the surface of the article to be treated during orafter dispensing said liquid mixture, to a second temperature; whereinthe second temperature is at least 30° C. greater than the firsttemperature.
 2. The method according to claim 1, wherein said article isa semiconductor wafer, and wherein said surface to be treated comprisesphotoresist that includes ions implanted during a previous processingstage.
 3. The method according to claim 2, wherein the semiconductorwafer is positioned on a spin chuck in a single wafer wet processingstation.
 4. The method according to claim 1, wherein the inorganic acidis an aqueous solution of sulphuric acid at a concentration of at least90 mass % or pure sulphuric acid (oleum).
 5. The method according toclaim 1, wherein the article to be treated is present in a processingchamber, the method further comprising introducing gaseous ozone intothe processing chamber.
 6. The method according to claim 1, furthercomprising heating the surface of the article to be treated during orafter dispensing the liquid mixture, to a temperature in excess of 50°C.
 7. The method according to claim 1, wherein the surface is heated toa temperature in excess of 100° C.